6 PHARMACEUTICAL BOTANY 



thickened by deposits of lignin, layer within layer until the cells are 

 often nearly filled with this substance. A lumen is found within the 

 centre of a stone cell which is in communication with radial pore canals 

 leading outward and in communication with the pore canals of adjacent 

 stone cells. Stone cells are distributed in fruits as gritty particles, 

 in barks and seeds. They are supporting structures. 



Fig. 2. — Stone cells from different sources, i, From coffee; 2, 3 and 4, from stem 

 of clove; 5 and 6, from tea leaf; 7, 8 andg, from powdered star-anise seed. (From 

 Steiens after Moeller.) 



Epidermis is the outer covering tissue of a plant and is protective 

 in function. Its cells may be brick-shaped, polygonal, equilateral or 

 wavy in outline. Their outer walls are cutinized (infiltrated with a 

 waxy -like substance called cutin). Among the epidermal cells of 

 leaves and young green stems may be found numerous pores or stomata 

 (sing, stoma) surrounded by pairs of crescent-shaped cells, called guard 

 cells. The stomata are in direct communication with air chambers 

 beneath them which in turn are in communication with intercellular 

 spaces of the tissue beneath. The function of the stomata is to give 

 off watery vapor and take in or give off carbon dioxide, water and 



